(CNN) -- Heavy rain and snow were falling over California on Monday, the first round in a series of storms poised to pummel the West Coast this week, bringing potential flooding and mudslides.

The El Niño-type storms forecast for the West could dump up to 6 inches of rain in some areas, according to CNN meteorologist Rob Marciano. Rain began falling Sunday night and was continuing Monday. A second round was expected later Monday night and Tuesday, with a third coming late Tuesday and into Wednesday.

In mountainous areas, 2 to 3 feet of snow is forecast. Heavy snow was already falling Monday. The storms are fueled by what is seen as a very typical El Niño pattern.

Flash flood watches were already in effect in southwestern California on Monday morning. The National Weather Service said coastal areas could experience 1 to 2 inches of rain near the coast and 2 to 4 inches in the foothills and mountains through Monday night. Maximum rainfall on some southwest-facing slopes could reach 8 inches, forecasters said. The rain could trigger mudslides, especially in recent burn areas.

The news is not all bad, forecasters said. The rain will help with severe to extreme drought conditions in much of California, and help build the snowpack in the Sierra Nevada.

However, areas like Las Vegas, Nevada, and Phoenix, Arizona, could see 3 to 4 inches of rain -- rainfall such areas are not equipped to handle, Marciano said.

The East Coast was also experiencing bad weather Monday, thanks to a winter storm that triggered heavy, wet snow. Some 10 inches of snow had fallen in Franconia, New Hampshire, as of Monday morning, Marciano said. Rochester, New Hampshire, received 7.3 inches. Snowfall totals could reach a foot in mountainous areas, he said.

A windstorm struck the Pacific Northwest overnight, and gusts of up to 60 mph knocked out power to thousands of households. The rain associated with that storm was tapering off Monday.